Electric vehicles and hybrid electric vehicles require onboard batteries to power their electric drive systems. The performance requirements of such vehicles generally require combining a large number of batteries together to provide a sufficiently high voltage and current for powering the electric drive system. For example, Li-ion batteries can be stacked to produce battery packs or modules with very high voltage and current capabilities. In conventional electric or hybrid electric vehicles, such Li-ion batteries often are configured to generate voltages on the order of 400V DC.
Typically, in such vehicle applications, the battery voltage is applied to the drive system under control of a battery management system (BMS) electronic controller. Specifically, the battery is connected to the vehicle via a contactor or other type of switch element controlled by the BMS. Throughout the lifetime of the vehicle, it is possible that the contactors may fail or degrade, resulting in improper operation. Such contactor faults or failures can arise from the very large operating currents and transients that are typically conducted by the contactors during operation of the vehicle. For example, currents in electric vehicles can exceed 200 A under some conditions. As a result, the switch elements used to connect the battery to the vehicle may become stuck open or closed under mechanical and/or electrical stress during operation.
When such contactor failures occur, it is generally desirable to generate control signals for the BMS such that the battery can be disabled to prevent further damage to the vehicle or to reduce the risk of injury to the driver or passengers of the vehicle. Further, it is generally desirable to generate signals such that service personnel can easily assess and repair the contactor failure.
Conventional methods for detecting the physical state of a contactor, a relay, or other types of switch elements in high voltage DC systems typically rely on providing a control loop based on coupling a waveform on a first side of a switch element and thereafter detecting the waveform on the second side of the switch element. For such applications, optocouplers have historically been used for several reasons. In particular, optocouplers have historically provided favorable droop, backswing, and common mode characteristics as compared to other coupling methods. Additionally, configuration of conventional optocoupler devices is well-understood. Further, optocoupler circuits have historically been less expensive and smaller than other types of signal coupling devices.
However, optocouplers have several disadvantages. First, the use of multiple optocouplers to couple and detect waveforms in a circuit generally requires fairly complex supporting circuitry. In particular, such circuits generally require additional power rails and circuit paths for operating multiple optocouplers. This not only increases overall circuit complexity, but also increases the chances of interference between components in the circuit and power requirements for monitoring the contactor. Second, the characteristics of optocouplers can vary during the operation and lifetime of the optocoupler. That is, as a function of time and/or temperature, optocoupler can exhibit variations in current transfer ratios and propagation delays, non-linear transfer characteristics, and other characteristics. Further, the light-emitting diodes (LEDs) in the optocouplers can degrade and wear out over time. Accordingly, the reliability and lifetime of optocouplers is typically limited. However, despite the limitations of optocouplers described above, optocouplers still remain the preferred means of coupling waveforms into a control loop.